1. Field of the Invention
The present invention relates to a laser oscillator and an optical communication method and system. The invention specifically relates to technology for scrambling optical signals, which can be used in a highly reliable optical communication system which invalidates illicit activities such as wire-tapping.
2. Description of the Related Art
In an optical communication system using optical fiber as a transmission medium, the communication can be tapped in the same manner as electrical communications using electrical wires by locally bending the optical fiber so that the mode of transmitting along the optical fiber is easily converted to an radiation mode, thereby enabling the light which is being transmitted in the optical fiber to be extracted to the outside. Furthermore, a communication system using spatial light propagation can easily be tapped by providing an optical receiver in the same manner as radio communications.
When the tapped signal can be demodulated, even in a case where the data has been enciphered, the possibility remains that the data can be theoretically deciphered.
For this reason, a desirable communication method is one in which it is difficult to demodulate a tapped signal. One conventional proposal for such a communication method utilizes quantum-mechanical fluctuations which cannot be artificially reproduced, such as the irregularity (chaotic nature) of a laser oscillator.
For example, in one communication method, laser light having chaotic fluctuation is used as the carrier, which data is added to and transmitted, and a part of the signal light transmitted is input a laser resonator on the receiving side so that the laser resonator can perform synchronous oscillation, whereby the carrier component is extracted and the data is subsequently demodulated. According to this method, without the aforementioned special means for extracting the carrier component, a signal detected by a conventional communication method for directly detecting ordinary signal light would always be distorted, making it impossible to demodulate the data.
When extracting the carrier component by inputting signal light to a laser resonator and performing synchronous oscillation, the light carrier frequency and the oscillating frequency of the laser resonator must be aligned with high precision. However, the optical frequency of infrared light used in optical communication is approximately 190 THz, which is much higher than the frequency of ordinary electrical and microwave communications (several GHz). Consequently, a large amount of change in the oscillating frequency is caused by fluctuation of the laser resonator (e.g. thermal and mechanical fluctuation of the resonator length). Since fluctuation reflects changes in the physical properties of the laser resonator, such as change in the refractive index, it has a wide band of components.
In attempting to inject laser light into another laser resonator and perform synchronous oscillation, the resonant frequency of the laser resonator must be aligned with great precision to the light frequency of the injected light.
However, the laser resonator attempting the synchronous oscillation has fluctuation in its own oscillating frequency similar to that of the other laser resonator. As a result, it is impossible to guarantee that the resonant frequency of the laser resonator attempting the synchronous oscillation will remain within the frequency bandwidth of the other laser resonator for a long period of time.
Furthermore, when the person performing the tapping has means for demodulating the signal obtained by synchronous oscillation as described above, conventional methods offer no physical means of stopping him from demodulating the tapped signal.